1. Field of the Invention
The present invention relates to a bias circuit providing an analog circuit with a bias.
2. Description of the Related Art
The gain of an amplifier circuit depends on mutual conductance (gm) of a transistor that is a part of the amplifier circuit, and on the load connected to the transistor. The mutual conductance (gm) changes depending on the manufacturing process of the transistor or the temperature of the transistor. Therefore, compensation of the mutual conductance (gm) is significant for compensation of the gain of the amplifier circuit. A circuit that compensates mutual conductance (gm) (hereinafter “gm compensating circuit”) is commonly known. According to “Design of Analog CMOS Integrated Circuits” (Behzad Razavi, McGraw-Hill Publication Co., Oct. 1, 2003, pp. 375-379), the amount of current running through a gm compensating circuit is, in light of the current mirror structure, the same as that running through a bias circuit that provides a source-grounded transistor of an amplifier circuit with a gate bias (gate-source voltage). As a result, the gate bias of the transistor is controlled and the mutual conductance gm is compensated so that the gain is compensated.
However, according to the conventional bias circuit, as the gm compensating circuit reduces the current running through the bias circuit when the gain increases, the bias point of a gate-grounded transistor connected to the drain terminal of the source-grounded transistor goes up in the amplifier circuit. Consequently, the output signal is distorted and the linearity is deteriorated when a signal of large amplitude is output from the drain terminal of the gate-grounded transistor (an output terminal of the amplifier circuit). Thus, the conventional bias circuit cannot obtain linearity and high gain of the amplifier circuit at the same time.